1. Field of the Invention
The invention relates to a method and an apparatus for the digital control of the phase of the system clock of a digital signal processing system which processes an analog signal containing a reference signal, wherein a fixed phase relation exists between the digitalized reference signal and the system clock.
Such methods are required, for instance, in clock systems for digital color decoding, such as according to the PAL or NTSC principle. In these cases, the system clock must be synchronized as to phase and frequency with a multiple of the color carrier frequency, since the sampling of the analog FBAS (color picture blanking synchronous or composite color video) signal and the further processing of the digitized sampling values is accomplished at the system clock frequency. For the further processing of the digitized sampling values in such a digital signal processing system, further clock signals at the color carrier frequency are to be derived from the system clock. Such signals have an exact phase relative to the reference carrier, which is the reference signal contained in the analog FBAS signal.
2. Prior Art
It is known from German Published Non-Prosecuted Application DE-OS No. 28 54 236 and British Pat. No. 2,028,138, that for this purpose the FBAS signal obtained in the usual way controls the auxiliary color carrier quartz oscillator, which is constructed as a square wave signal clock generator, as in known color television receivers. The clock generator is synchronized by the color synchronization signal contained in the FBAS signal (a so-called burst signal) with a multiple of the auxiliary color carrier frequency. In this solution to the problem, the analog FBAS signal is therefore fed on one hand to an analog-to-digital (A/D) converter with a sampling apparatus and on the other hand is fed parallel thereto to an analog phase control loop which uses the burst signal contained in the FBAS signal for synchronization.
In digital signal processing systems, such solutions to the problem have the disadvantage of not ensuring the desired phase, because in the A/D converter propagation times occur between the input and the sampling device as well as in the clock generator. These are not negligible and they shift the sampling instant. Correcting through the use of a variable propagation time element is expensive and is not stable over the long term. It is a further disadvantage that such an analog phase control loop must be integrated together with the digital circuits on a chip, in a technology which is optimized for digital circuits.